1. Field of the Invention
The present invention relates to a method and apparatus for controlling interference in a cellular communication system. More particularly, the present invention relates to a method and apparatus for controlling inter-cell interference in a base station of a cellular communication system in which a frequency reuse factor is 1.
2. Description of the Related Art
With the recent development of next-generation mobile communication systems, technologies to increase frequency transmission efficiency or spectral efficiency are under study in many aspects. It is widely known that the frequency transmission efficiency of a cellular communication system is maximized when a frequency reuse factor is 1. However, when the frequency reuse factor is 1 in the cellular communication system, the same frequency channels are reused in all base stations, causing inter-cell interference. More particularly, in urban areas where a required amount of data is relatively large, as the cell size is reduced, the inter-cell interference problem is serious.
In this case, an interference-limited environment occurs in which even though base stations increase their transmit power, the system capacity does not increase due to a corresponding increase in the inter-cell interference. In order to increase the frequency efficiency in this interference-limited environment, it is important to effectively control the inter-cell interference.
The inter-cell interference is closely related to a power/resource allocation pattern transmitted by each base station. Therefore, an inter-cell interference pattern varies depending on a power/resource allocation scheme used by each base station's scheduler. To effectively control the inter-cell interference, it is important to coordinate resource scheduling and allocation schemes among neighbor base stations. For this coordination, a Fractional Frequency Reuse (FFR) technology has been proposed. The FFR technology refers to a technology of dividing, for example, one cell (or sector) into two or more areas and setting a different frequency reuse factor in each of the divided areas. For example, in a case where the FFR technology is applied, the frequency reuse factor is set to 1 in an inner area (e.g., a cell center area) adjacent to the base station in the cell because an influence of the interference is relatively small, and the frequency reuse factor may be set to a value greater than 1 in an outer area (e.g., a cell boundary area) adjacent to the cell boundary because an influence of the interference is relatively large.
FIG. 1 shows an example of a frequency allocation pattern in a cellular communication system using an FFR technology according to the related art. In this example, the cellular communication system is a 3-sector system using the FFR technology.
Referring to FIG. 1, when the FFR technology is applied, first resources 110 with a frequency reuse factor of 1 among all frequency resources are mainly allocated to users located in the cell center area where they are scarcely affected by inter-cell interference even though 3 neighbor sectors use the same frequency resources. Frequency resources 121, 123 and 125 allocated so as not to overlap between neighbor sectors among all frequency resources, e.g., among second resources 120 with a frequency reuse factor of 3, are mainly allocated to users located in the cell boundary area where they are significantly affected by inter-cell interference.
Use of the FFR technology may improve a Signal-to-Interference and Noise Ratio (SINR) in the cellular communication system, and improve throughputs of users in the cell boundary area, thereby contributing to an improvement of the overall system efficiency.
Among the second resources 120 in FIG. 1, the remaining resources, except for the frequency resources 121, 123 and 125 allocated to sectors α, β and γ, are not allocated so as mitigate interference to users in cell boundaries among the neighbor sectors. In the FFR technology, since the remaining resources are not used, available frequency resources are reduced, causing a decrease in the average sector capacity. To solve these problems, a so-called soft FFR technology has been proposed that uses the remaining resources unused in FFR as power-restricted resources that use little power, thereby using all available frequency resources while mitigating interference to the users in the cell boundaries between the neighbor sectors.
FIG. 2 shows an example of a frequency allocation pattern in a cellular communication system using a soft FFR technology according to the related art.
Referring to FIG. 2, reference numerals 210 and 230 correspond to the first resources 110 with a frequency reuse factor of 1 and the second resources 120 with a frequency reuse factor of 3, respectively, which have been illustrated in FIG. 1. The soft FFR technology allocates the remaining resources 222, 224 and 226 excluding frequency resources 221, 223 and 225 allocated to their associated sectors α, β and γ from the second resources 230, as the power-restricted resources, thereby minimizing the reduction in the average sector capacity while improving throughputs of the users in the cell boundary areas.
The above FFR technology and soft FFR technology should determine inter-sector frequency allocation patterns during installation. Thus, the network manager should plan and set frequency allocation patterns for FFR and a restricted power level of the power-restricted resources according to the environment during network installation. After the network installation, if the cell environment is changed, the network manager needs to re-perform cell planning and manually change the frequency patterns.
Even though there is no change in the cell environment, if users are concentrated in the cell boundary area or the cell center area, it is necessary to instantaneously change the frequency allocation patterns according thereto. For example, if it is assumed that users are concentrated in the cell center area, it is optimal to operate the frequency reuse factor as 1 by using all frequency resources and power because all users in the cell are less affected by the inter-cell interference.
As another example, if it is assumed that all users are located in the cell boundaries, it is necessary to reduce resources allocated to the area with a frequency reuse factor of 1 and increase resources allocated to the area with a frequency reuse factor of 3, because all users in the cell are significantly affected by the inter-cell interference.
However, since the FFR technology and soft FFR technology use frequency allocation patterns that have been previously set by performing cell planning, it is not possible to control the inter-cell interference while adaptively varying resource allocation patterns according to the instantaneous change in the cell environment and/or the distribution of users.